Morphology of the coating
The protective layer that is created on the products as a result of hot galvanizing is the result of the diffusion of zinc at the temperature of the bath, through the most superficial layer of the steel. The term reaction to indicate the set of processes that lead to the formation of the coating is now universally accepted. It is not a chemical reaction, but a kind of metallurgical reaction, a physical process. At the surface of the steel there is an exchange between the two phases which gives rise to the formation of layers of alloys with different compositions of the two metals iron and zinc. For this reason, the zinc coating is “welded” on the steel surface, with obvious benefits compared to other anticorrosive treatments that involve overlapping of metals (such as electroplating or metallization processes) or organic coatings (liquid or powder paints. ).
The iron / zinc alloys developed during immersion in the galvanizing bath are well characterized and recognizable by their composition and crystalline structure. Each of them, in fact, corresponds to one of the homogeneous phases foreseen by the iron-zinc state diagram (ie of “solubility”). Their succession shows an increasing zinc content towards the outside. In a typical galvanizing coating, starting from the steel substrate, the γ (gamma) layer with a thickness of about 1μm can be recognized, in which zinc is present for about 70% (the percentage of iron varies between 26.8 and 31.1 %). The subsequent δ layer (delta) contains an amount of iron of the order of 10%. In the following ζ (zeta) layer, 7% iron is present. In the photos under the microscope, the crystals of layer ζ oriented upwards, oblong and perpendicular to the surface are clearly recognizable.
In most cases, albeit with significant exceptions as will be illustrated below, in the galvanizing coating there is a last and outermost surface layer, called the η layer (eta), which is made up of zinc with a composition coinciding, in practice, with that of the bathroom. It is the result of the last interaction with the molten zinc before the extraction of the pieces and is deposited by dragging. For traditional baths, it is almost pure zinc, as, at room temperature, it has a maximum iron content of approximately 0.008%.
The case of a galvanizing bath consisting of a technological zinc alloy (with the addition of tin and nickel, for example) is different, in which the η layer will have a composition obviously influenced by the presence of the other elements in the alloy. Note that the zinc bath according to the Italian and international standard UNI EN ISO 1461 certainly cannot contain less than 98% zinc.
Mechanical properties of the coating
The fact that the coating is made up of layers with different compositions implies a certain variability of the mechanical properties along the thickness. As for the mechanical hardness, the innermost alloy layers are harder than the substrate steel, reaching values around 240Hv (hardness expressed in Vickers). Just to fix the ideas, consider that the most common mild steels generally have hardnesses around 160Hv. Finally, the superficial zinc is “softer” and therefore is able to absorb any blows, acting as a sort of shock absorber. The underlying layers of greater hardness give the galvanized steel advantages relating to surface hardening in terms of wear resistance and toughness.
The fact that galvanizing does not consist in a simple deposition or addition of metal on the surface of the steel, represents one of the distinctive characteristics of this anticorrosive treatment: since the layers of the coating alloy are the result of an interpenetration between steel and zinc , they are intrinsically linked to the steel support. It is very difficult to completely remove the coating and even in the event of severe damage, with apparently total detachment, a lower layer of ferroalloy always remains on the surface, still providing the electrochemical protection determined by its zinc content, in addition to the sacrificial cathodic action of zinc. present on the areas proximal to the fault.
When galvanizing thicknesses do not exceed 80-100μm, coatings are obtained that are able to withstand the greatest stresses, especially those of an impulsive nature. In the case of higher coating thicknesses, the corresponding leverage effect clearly leads to a disadvantage. In any case, the adhesion of the layer obtained with hot galvanizing is higher than that possible with other protective coatings or paints. The need to offer an ever better product and in terms of homogeneity and surface uniformity, together with the need to reduce the problems caused by the excessive thickness of the coating, it has favored an in-depth research for the understanding of the growth mechanisms of the zinc-iron alloy layers and the development of control techniques.
Factors influencing the growth of the coating
The mechanism of formation of the zinc coating is mainly influenced by the temperature and composition of the bath, by the contact time between the element to be galvanized with the molten zinc and by the composition and surface state of the steel. The presence in the steel to be galvanized of elements such as, for example, carbon, silicon and phosphorus, is crucial for the formation and growth rate of the galvanizing layer during immersion.
Numerous studies have investigated the speed of the iron \ zinc metallurgical reaction, leading to the definition of a law such as:
- W = K (T) tn (T)
- W = rate of reaction. W is a measure of the amount of iron that, per unit area, reacts with zinc to form the alloys that make up the coating
- K = dimensionless coefficient, which depends on the temperature and reactivity of the steel
- t = dive time
- n = characteristic exponent of the type of reaction dependent on the temperature.
In fig. 5.5, it can be observed that, starting from the melting temperature of the zinc towards increasing values of the bath temperature, there is an interval (area highlighted with blue), in which galvanizing can be successfully obtained. A thoughtful choice generally leads to choosing operating conditions between 440 and 460 ° C, which allows temperature variations inside the bath, without significant effects on the growth of the layer. Under different conditions, there may be “anomalies” in the composition and crystalline structure.
However, technological uses of high temperature baths are also possible. In this case, 550 ° C is reached. The formation of the ζ (zeta) layer no longer takes place and, therefore, the coating is composed of a mixture of δ (delta) phase crystals and zinc. It is difficult to obtain coatings with a thickness greater than 100μm at such temperatures. In general galvanizing, the immersion time of the pieces in the usual conditions is generally included in the interval between 1.5 and 5 minutes, depending on the more or less linear shape of the products, and the thickness of the profiles with which they are assembled. It is in fact in these first minutes that the greatest growth in thickness occurs. Particularly complex elements may, however, require the immersion to be extended beyond 10 minutes.
In fact, the thickness of the steel plays a decisive role in determining the residence time of the product inside the galvanizing bath, as already stated in the previous chapter. The thicker profiles require a longer time to uniform their temperature to that of the bath and during extraction they keep warmer and this positively affects the kinetics of formation of the layer. The roughness of the surfaces can also significantly affect the thickness of the coating, due to drag effects and the increase in the specific surface of the steel exposed to the action of zinc. In some cases, the effect is more evident, as occurs for very rough pieces because they are sandblasted with particularly angular means or with pieces that originally have very corroded surfaces before pickling.
Reactivity of steels: influences of the substrate composition
The differences in the composition of the steel, due to the technological addition of metals or other elements in addition to the iron and carbon in the alloy, lead to a greater or lesser increase in thickness or, as is commonly said, greater or lesser reactivity.
In fact, steel is an alloy consisting not only of iron and carbon. Other elements can be added both to confer particular properties to the alloy and to facilitate phases of the production process. These constituents, although present to a minimal extent, influence the formation and growth processes of the galvanizing layer according to quantity and type, consequently varying its composition, properties and appearance.
The galvanizing layer, during contact with the molten zinc, can proceed even without the formation of all the phases in fig. 5.2, from the ferroalloy up to the η (eta) layer, or with predominant growth of one phase over the others. Light coatings can form with a predominance of the η (eta) layer, which are more plastic and with a lower iron content. This is achieved, for example, in the galvanizing of aluminum-quenched steels, which inhibits the reaction, preventing it from proceeding at the expense of layer η, once the product is extracted from the galvanizing bath. These steels can allow the realization of bright zinc plating and with a very pleasant appearance, but have lower thicknesses. For high aluminum contents, it can even become difficult to obtain coatings that meet the minimum thickness requirements set by standards and specifications. Conversely, other elements, such as silicon, involve heavy coatings, characterized by the presence of harder and therefore more fragile iron / zinc alloy states, as we will see below. In some cases, if the concentrations of these elements reach critical values, the coating can present itself with characteristic non-uniformities and anomalous excess thicknesses.
As for the differences in structure and alloy in the zinc layer on the manufactured articles, it should be noted that even if in some cases, as the composition of the steel substrate varies, coatings are obtained with differences in the distribution of the iron / zinc phases, the properties anticorrosive are never decisively influenced by this. In other words, with the same environmental conditions, the duration of the protection is proportional to the thickness of the coating, in general. The performances are practically the same whether it is a pure η phase zinc coating or a succession of different iron-zinc alloy layers. All commonly used steels can therefore benefit from the protection offered by galvanizing to the same extent.
At the end of the galvanizing of a construction element made by assembling pieces of steel of different origins, it is possible to observe the formation of a coating with a different appearance on the parts of different composition, due to the different behavior of the steel during the iron-zinc reaction. Moreover, the difference in appearance alone does not affect the anticorrosive properties of the coating, which is not influenced by the characteristics that determine the color and brilliance.
With the exception of some cutting steels, with a high sulfur content, all steels on the market are suitable for receiving zinc plating.
Influence of silicon and phosphorus on the galvanizing of steels
In daily practice, the elements present in steel with the most significant influences on the galvanizer’s work are silicon and phosphorus. Researchers in the sector have long analyzed its effects.
It has been observed that the presence of silicon in quantities between 0.03 and 0.12% (Sandelin range) or higher than 0.25% (hyper-Sandelin steels) is able to accelerate the iron-zinc reaction. The resulting coating has a visibly greater thickness. In cases of very pronounced reactivity (maximum peaks in the diagram in fig. 5.7 or for steels with a very high silicon content), the product after immersion in zinc may show a uniform greyish or patchy appearance, even localized in specific areas. In Sandelin or hyper-Sandelin steels (respectively in red and yellow in fig. 5.7), in fact, it may happen that the entire coating is composed exclusively of iron-zinc alloy layers, completely missing the outermost layer of pure zinc. Given the enhanced reactivity, the reaction between zinc and iron can continue with appreciable speed even after extraction from the galvanizing bath. The ζ (zeta) layer continues to grow at the expense of the η (eta) layer as long as the temperature of the galvanized piece exceeds 150-200 ° C. The different possible colors from light gray to dark gray, and the opaque or bright appearance, are due to the different composition of the iron / zinc alloy emerging.
As previously stated, a thicker zinc coating has a lower mechanical resistance to impulsive stresses (impacts), tends to detach more easily, but protects the underlying steel more effectively from corrosion, given the presence of higher quantities of zinc. high. Generally, the standards do not prescribe upper limits for the thickness of the coating, but it is advisable that it does not exceed 160 ÷ 180μm. There are special cases in which, despite being anomalous, thicknesses greater than 200 ÷ 220μm are also acceptable, especially if the products are very heavy, provided that the increased thickness does not involve problems with the installation, assembly or function that they must carry out.
For the purpose of determining the reactivity, it is also good to evaluate the amount of phosphorus, sometimes deliberately added to the steel to improve some mechanical characteristics. In fact, even for steels with silicon contents lower than 0.03% or between 0.12 and 0.25%, the combined effect of silicon and phosphorus can influence the galvanizing process, amplifying the Sandelin effect.
An empirical formula useful for predicting whether a steel has a Normal activity or not is as follows:
Yes + 2.5 P ≤ 0.09%
That is, the sum of the total phosphorus content multiplied by 2.5 and the silicon content must be less than or equal to 0.09%.
From experimental evidence, it appears that phosphorus hinders the formation of the δ (delta) layer in favor of the ζ (zeta) phase. The γ (gamma) layer, on the other hand, becomes discontinuous. In some steels, such as those suitable for automatic cutting, sulfur concentrations higher than 0.2% may exist. Such high quantities can accelerate the galvanizing reaction to the point of transforming the process into an aggression of the steel by the zinc. Similarly, manganese, chromium, nickel, niobium, titanium and vanadium can also increase the reaction rate. However, at normal concentrations their effect is completely irrelevant.
Classification of steels based on silicon and phosphorus content
For products that require coatings with overall characteristics of the highest quality standard, for particular needs regarding the uniformity of the coating and the bright appearance of the pieces, three classes of chemical composition of the steel can be defined according to the subdivision represented by the graph in fig. 5.8.
For what has been said, there are numerous factors that determine the reactivity of the steel and, therefore, the final appearance of the galvanizing.
The classes that we define below represent a pure guideline that aims to suggest compositions that increase the probability of obtaining the desired appearance. Galvanizing can be obtained with more than satisfactory results even for steels that do not fall within the indicated composition ranges.
Class 1 (for Si ≤ 0.03%, P ≤ 0.025% and Si + 2.5 P ≤ 0.09%)
The coatings obtained on the steels of this class normally have the structure constituted by the succession of the phases foreseen by the binary diagram Fe-Zn. They have a thickness between 60 and 100 μm and have a brilliant appearance. Some problems may exist when the Si and P contents are lower than 0.01%, since the thicknesses that can be obtained may be lower than what is prescribed by the regulations. Even for steels belonging to this class it is possible that abnormal coatings are produced (high thicknesses, dispersed phases). These anomalies are found on low thickness cold rolled and / or glossy finish profiles, when the phosphorus is greater than 0.015 – 0.020% and the aluminum exceeds 0.04%.
Class 2 (for Si ≤ 0.04%, P ≤ 0.025% and Si + 2.5 P ≤ 0.1025%)
The steels belonging to this class are more reactive than those of the previous class. However, for Si and P concentrations far from the maximum limits, the coating retains the characteristics obtained for class 1 steels. minutes. The thickness for this class varies from 100 to 150 μm and the appearance can go from bright to opaque-light gray. In addition, some surface irregularities may occur, such as streaks and point heaps.
Class 3 (for 0.15 ≤ Si ≤ 0.20%, P ≤ 0.02% and Si + 2.5 P ≤ 0.25%)
Steels with medium-high reactivity belong to this class. The structure of the coating is dispersed phase with dendritic-structured ζ-phase crystals, especially for P contents ≥ 0.015. It is possible that the presence of phase η (pure zinc) occurs on the surface, based on the residence times in the zinc bath and the galvanizing temperatures. For the latter class, the thickness belongs to the 120-220μm range and the appearance varies from bright to opaque-dark gray. For higher thicknesses the coating can be brittle.
Layer growth control
It must be said, however, that given the presence of many factors influencing growth, which are not always controllable, no galvanizer is able to accurately predict how much zinc will be applied to the product. It is also true that the greatest problem is the containment of the growth of the layer, rather than the presence of any difficulties in obtaining satisfactory thicknesses. However, it is very unlikely, but not impossible, that an artifact may present obstacles to obtaining the thicknesses required by the specifications. This phenomenon can be highlighted with the existence of various causes (for example, cold rolled steel, calmed with aluminum, of small thickness and with a surface not subject to oxidation). Except for really extreme cases, the galvanizer, based on his experience, can implement particular procedures to solve the problem (for example, longer stay in pickling or resorting to sandblasting of the pieces, to increase their roughness).
Composition of the galvanizing bath
The zinc bath, by rule pure at least at 98.0%, it usually also contains a very small amount of other metals that are added as technological components of the alloy or constitute impurities of the zinc introduced. The presence of small quantities of other elements in addition to zinc is necessary to improve the aesthetic quality of the finished product, to control the growth of the thickness of the coating, the uniformity of the layer and, in some cases, to improve corrosion resistance. A reference composition of the bathroom can be (but there are many variations from system to system):
- Zinc & gt; 98.0% by weight
- Lead 1.0% by weight
- Iron 0.03% by weight
- Aluminum 0.002% by weight
Traces of other metals
In some cases, always remaining within the maximum limit of 2% total of elements other than zinc, alloys can be found with:
- Tin up to over 0.4% by weight
- Nickel up to 0.06% by weight
- Bismuth up to over 0.1% by weight.
In general galvanizing, aluminum and lead are added for their influence on the thickness and the brighter exterior appearance of the coating. The addition of lead affects the physical characteristics of the zinc, in particular the viscosity and surface tension. This helps the zinc to wet the steel during galvanizing and, making the alloy more fluid, facilitates the drainage of excess zinc from the surface of the steel during extraction from the tank.
Lead can also be used to protect the tank and is very useful in the cyclical operation of extraction of the mattes. In fact, they have a higher specific weight than zinc and tend to settle in contact with the lower part of the tank. In the presence of a deposit of molten lead, precipitated on the bottom of the tank, the mattes float there. In this way, a thickness is created between the mattes and the tank, which allows the special buckets to slip in, removing them without the risk of hitting and damaging the bottom of the tank.
Effects of the bath composition on reactivity
In order to improve the quality and appearance of galvanizing, in recent years we are witnessing the development of alloys for galvanizing characterized by the use of other elements added to the zinc bath in very limited quantities, so that what is required is respected. by the UNI EN ISO 1461 standard on global purity (98% zinc). This led to the development of hot dip galvanizing alloys with different formulations and different contents mainly of nickel, aluminum, tin, bismuth and magnesium.
Let’s briefly analyze the effects of these elements:
Nickel: to limit the consequences of the presence of silicon (in particular when the concentration in the steel is in the range 0.04 – 0.12%) a galvanized bath alloyed with nickel is used. If you add up to a maximum of 0.06-0.08% to the bath, you get a certain control on the growth of the layer even for higher silicon contents in the steel. In this case, it is possible to obtain a thickness reduction of about 12 – 15% on steels with Sandelin and hyper-Sandelin silicon concentrations, without altering the corrosion protection properties at all.
Aluminum: About 0.005% is added to the bath. It determines the brilliance and uniformity of the layer. A not secondary aesthetic effect is the reduction of the classic flowering, with the decrease in the size of the characteristic sequins and the reduction of the contrasts of the relative edges. The addition of aluminum, with some precautions carried out on the flushing bath, can be brought to percentages of 0.04%. Although at the minimum concentrations reported, aluminum affects the development of the layer with an inhibiting action, so it must be added to the bath very carefully. Larger quantities can create difficulties in the development of the layer and cause undesirable reactions with the flushing salt. Alloys with a higher aluminum content have been proposed (of the order of 1-2%, up to 5%), but they require radical changes in the process, with the replacement of the traditional flushing operation with different treatments, such as for example the cementation or the electrodeposition with copper of the surface of the steel to be galvanized. In these processes, currently at an experimental stage, the coatings obtained show very thin thicknesses and anticorrosive properties substantially different from traditional galvanizing. If one day they were adopted in an industrial practice, they would be destined for particular applications, different from the protection of structures, articles and furnishings that are today hot-dip galvanized. These are alloys designed for use, mostly, in the automotive industry, in which, due to the need for Inspecting tolerances, the thickness and uniformity of the coating must be checked
Tin: with the opposite effect to aluminum, as regards the development of flowering is the addition of tin, which accentuates it. In some cases, this may be required for aesthetic reasons. In addition, tin is a fluidifying agent. In general, in Italy, percentages do not exceed 0.4-0.6%. Some bathrooms are alloyed with tin up to more than 1%. In this case, the galvanizer must apply great caution due to the aggressiveness that the alloy shows towards the galvanizing tank. Additions of tin with a high content lead to a relative increase in the hardness (and brittleness) of the galvanizing layer. The anticorrosive properties are not much affected
Bismuth: new formulations of galvanizing alloys involve the use of a small amount of bismuth (0.1-0.2%) as a fluidifying agent. It can be added as a partial replacement for lead. In these cases, the galvanizing bath can perform its function in the presence of very low lead contents (≈ 0.1) or, at most, in its absence. It favors the control of the growth of the layer
Magnesium: to counteract the tendency to the formation of white rust, a small amount of magnesium (up to 0.03%) is added to the bath.